Compounded fluoborate chemical surface treatment of aluminum and aluminum alloys



Oct. 13 1953 B. w. FLoERscH coMPouNDED FLuoBoRATE CHEMICAL 2'655'439 suRFAcE TREATMENT oF ALUMINUM AND ALUMINUM ALLoYs Filed nec. 15, 1952 IOO Tm. of Solution TI' F.

I I I I I I I I I I I Surfac l Resistance Micro ohm:

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20 Efching Timo in Mlm. INVENTOR' BERNARD W. FLOERSGH AGENTS Patented Oct. 13, 1953 COMPOUNDED FLUOBORATE CHEMICAL SURFACE TREATMENT OF ALUMINIM AND ALUMINUM ALLOYS Bernard W. Floersch, San Diego, Calif., assignor to The Ryan Aeronautical Co., San Diego, Calif., a corporationof California Application December 15, 1952, Serial No. 326,127

Claims. 1

This invention relates to the chemical surface treatment of aluminum and aluminum alloy material to remove oxides and to remove the lead and zinc contamination encountered in drop hammer forming as a result of the use of lead punches and zinc alloy dies in such forming.

The primary object of this invention is to remove the oxides and contamination in order that the metal surface be prepared for electrical resistance welding and possibly other processes.

Another object of this invention is to provide a process which overcomes the defects encountered in the use of presently known processes such as the use of fiuosilicic used alone or With wetting agents. By way of explanation, the defects are principally over-etching, non-uniformly low surface resistance, and short holding time. With regard to the over-etching, it is pointed out that this over-etching tends to be selective So that a non-uniform surface is the result. It is true that a more uniform surface is attained When the etching is limited to a time period of 12 to 15 minutes, but this time is often insuificient to complete the removal of the lead and zinc from the surface of the material. The high resistance, or non-uniformly low resistance, is the result of the incomplete removal of the contaminating metal. The holding time is that period during which there is no change `in the surface resistance of aluminum after the same is removed from the etching solution, and it has been found that this holding time in the case of etches heretofore proposed, at least in the case of the fiuosilicic acid, rarely exceeds 12 hours. The instant invention overcomes each of the said defects. In addition, the present invention overcomes the tendency for a black deposit of finely divided iron to form on the material. Certain proportions of the copper in the aluminum alloys results in the deposition of copper on the surface of the material due to galvanic action, and when fiuosilicic acid is used the deposition of iron and copper, generally termed smutting, is a serious problem, Whereas with the present invention such smutting is completely avoided.

Another important object of this invention is to provide a process which is very speedy, a time lapse of two minutes being usually sufiicient for ,satisfactory immersion. I n this regard, it is also very important that the desired low surface resistance of approximately zero microhms is attained in two to four minutes and again after a twenty minute immersion time. In general, commercial processes now used exhibit an increase in surface resistance after the Optimum immersion time has been exoeeded. However, under ordinary circumstances the resistance will not increase rapidly after a twenty minute period in the use of the presently disclosed process.

In the drawing accompanying this specification and constituting a part of this disclosure, the single figure is a graph showing the surface re-l sistance of material plotted against etching time.`

In brief, the etching compound is comprised of only three active substances, two de-oxidizers and`4 a mild oxidant. F'luoboric acid, provided by adding a fiuoborate, is the main etchant, nitric acid;` is the secondary de-oxidant, and chromic acid is. the mild oxidant and metal complexer. The criteria used in determining the proper composi-u tion of the present etch may be thought of as. the zero surface resistance to be achieved. Thereason for this is that weldability or good penetration and shear strength is dependent on low; surface resistance. The complete procedure for' 'treatment of 61S aluminum consists of a fiveminute treatment in alkaline precleaning solution, followed by a two to four-minute etch using the etching compound diselosed immediately below, and then followed by a one-minute post-etch in 1.8% solution, by weight, of fluosilicic acid. The etch consists of the following ingredients in the indicated percentages by weight:

Per cent byweight Chromic acid 0.1 to 1.0 A source of fluoborate anion 0.5 to 6.0 Nitric acid 2.0 to 12.0

The foregoing useful ranges of the components of the etch are obviously not particularly limiting, and this fact Will be recognized as important since it means that the etching fiuid can be used for a considerable time without Constant checking of the percentage composition thereof. Furthermore, the immersion time is not unduly critical and the material may be removed after two or three minutes immersion or after a twenty minute period if more convenient. This is true: because the surface resistance does not change materially during the longer period of immersion and no smutting or plating with copper is experienced.

The source of fluoborate -anion may be fluoboric acid, ammonium fluoborate, a fluoborate salt of ammonia or alkali metal such as sodium fluoborate, the last mentioned being possibly preferable in most instances. The graph in the accompanying drawing will be self-explanatory, thesurface resistance being reduced to approximately zero microhms in two to four minutes and again reaching this low resistance after an immersion aesaso period of approximately twenty minutes. It is noteworthy that the time of immersion can be extended considerably beyond the twenty minute period, shown on the graph, without an increase in surf-ace resistance. After 96 hours the surface resistance 'does not exceed 50 microhms. ilinally, it may be mentioned that the instant Vprocess was primarily developed for preparation of aluminum and aluminum alloy material for resistance welding, but it is conceivable that other uses may be found for the process.

In Summary, the etch produces .a uniform, low resistance surface faster than most etches, is unaifected by small temperature changes, at 'least between the limits of 65 'Ft to 1805 'retards over-etching', possesses a long holding time, and prevents the smutting or'pl'ating'of copper afn'l iron on the surface of the material.

Further description would appear to bo unnecessary.

It'is understood lthat m'inor 'Variation Tfro'm the form of the invention di'sclosed herein Ina'y be made 'without departur'e from the spirit and scope of 'thein'ventionrandthat the 'specification and drawin'g are to be considered as' merel'y illustrative rather than limiti'n'g.

I 'claim-1 l. `A pr'ocss'fb'r ul'face 'treatment of `ali'lr'r'l-fi'iim and -aluminum alloy material to 'remove oxides and surfacecontamination-encountered in forming Operations, comprising pre-cleaning the material with an valkaline solution, then :immersing the m'aterialin an 'etch co'n'sisting'of .-1% to 1.0% of chromic acid, 045% 'to '6.0% of 'sodium fluoborat'e 'and 2.01% to 12.0% of nitric acid, said percentages being by weight, andthen post-etching the material with a solution of fluosilicic acid.

2.- A process according to claim 1 wherein the pre-cleaning step is of approximately five minutes duration and the 'post-'etchi'ng 'step is of approx'- imately one minute duration.

3. A process according to claim -2 wherein the percentage by weight of the fluosilicic acid said post-etching solution is 1,8%.

4. A process according to claim 1 wherein the time of saidimmersion'in 'the 'etch is of the order of three minutes.

5. A process for surface'treatment of aluiniinum and aluminum alloy material to remove oxides 4 and surface contamination encountered in forming Operations, comprising immersing the material in an etch consisting of 0.1 to 1.0% of chromic acid, 0.5 to 6.0% of sodium fluoborate and 2.0 to 12.0% of nitric acid, said percentages being by weiht. ,I

:6. A process of `preparing 'aluminium and aluminum alloy material for Welding, comprising immersing the material in an etch consisting of 0-.1 to 1.0% chromic acid, 0.5 to 6.0% of a source of fluoborate fan-ion and 2.0 to 12.0% of nitric acid, said percentages being by weight.

7. A process of preparing aluminum and alumi'nfim allofyr material for welding, comprising im'rne'i'sin the material in an etch consisting of '0.1 to '110% Jc'h'ronii'c acid, 0.5 to 6.0% of a source of fluoborate anion 'and 2.0 to 12.0% of nitric acid, said percentages being by weight, for approiilmately three minutes.

8. A process of preparing aluminum and aluminu'm alloy material for welding, comprising pre-'Cleaning the material in an alkaline solution, then iimmersing the material in an etch consisting of 0.1 to 1.0% chromic acid, 0.5 to '6.0% 'of a source of 'fluobor'ate ianion and 2.0 to 12.0% of nitric acid, said percentag'es 'being by weight, and then 'post-etchin'g the material With a solution of fluo'silicic acid. i

9. A process of 'preparing aluminum and alumin'u'm allo'y material for welding, comprisin'g immersing the material in 'an etch consisting of 0.1 'to '1.0% chromic acid, 0.5 to 6.0% of Vsodium fiuborate and 2.0'to V12.0% of nitric acid, said percenta'ges being' by Weight.

10-.A process of preparing aluminum and alumiinum alloy material for welding, comp'rising i'n'i-nfiersing` the material in a'n etch 'consisting of 0.1 to v1.0% chromic acid, 0.5 to 6.0% of sodium fluoborate and 2.0 to 12.0% 'of nitric acid, sai'd percentages being by weight, for at least two minutes at ambient temperature.

BERNARD W. lrffloir'asCH.

References Cited in the file of this patent ITED s'rA'rEs PATENTS Number Name Date 2.465,750 Reid e -..a- Mar. 29.., 1949 2.637,634 Howard et al. May 5, 1953 

1. A PROCESS FOR SURFACE TREATMENT OF ALUMINUM AND ALUMINUM ALLOY MATERIAL TO REMOVE OXIDES AND SURFACE CONTAMINATION ENCOUNTERED IN FORMING OPERATIONS, COMPRISING PRE-CLEANING THE MATERIAL WITH AN ALKALINE SOLUTION, THEN IMMERSING THE MATERIAL IN AN ETCH CONSISTING OF 0.1% TO 1.0% OF CHROMIC ACID, 0.5% TO 6.0% OF SODIUM FLUOBORATE AND 2.0% TO 12.0% OF NITRIC ACID, SAID PERCENTAGES BEING BY WEIGHT, AND THEN POST-ETCHING THE MATERIAL WITH A SOLUTION OF FLUOSILICIC ACID. 